This invention improves upon current techniques for manufacturing high impedance surfaces which surfaces are also known as resonant textured ground planes or a xe2x80x9cHi-Zxe2x80x9d surfaces and which surfaces are presently made using printed circuit board techniques. The present invention provides new methods of manufacturing such surfaces based on molding and/or related techniques, and also provides several structures that are manufacturable using these techniques. The invention allows Hi-Z surfaces to be mass-produced more rapidly and at a lower cost than the prior art techniques, which primarily involve printed circuit board technology. This invention also provides a Hi-Z structure in which the capacitors are vertical, instead of horizontal, so that they may be trimmed after manufacturing, for tuning purposes.
Recently, a new kind of electromagnetic ground plane has been developed which is known as a high-impedance or Hi-Z surface. See D. Sievenpiper and E. Yablonovitch, xe2x80x9cCircuit and Method for Eliminating Surface Currents on Metalsxe2x80x9d U.S. provisional patent application, Serial No. 60/079,953, filed on Mar. 30, 1998 by UCLA and a related PCT application published as WO 99/50929 on Oct. 7, 1999. This prior art structure consists of a metal ground plane covered with an array of tiny resonant cavities. These resonant cavities alter the effective electromagnetic impedance of the surface, so that it appears to have a high impedance ( greater than  greater than 377 ohms), instead of a low impedance (≈0 ohm) like an ordinary metal surface. Because of its high impedance, the Hi-Z structure can support a finite tangential electric field at its surface, which is not possible with a smooth metal ground plane. This textured surface is important for various applications in the field of antennas. In particular, it is useful for low-profile antennas because radiating elements can be placed directly adjacent to the Hi-Z surface (i.e. spaced less than  less than  less than 0.01 wavelength therefrom) without being shorted out. This provides an advantage compared to an ordinary metal ground plane, which normally requires a separation of roughly xc2xc wavelength between the ground plane and the antenna, resulting in antennas that are at least xc2xc wavelength thick. In addition to providing a way to produce very thin antennas, the Hi-Z surface also suppresses surface currents, which tend to interfere with the performance of the antenna by propagating across the ground plane and radiating from edges, corners, or other discontinuities. The radiation produced by these surface currents combines with the direct radiation from the antenna, and produces ripples in the radiation pattern, as well as significant radiation into the backward direction behind the ground plane. By suppressing these surface currents, one can produce antennas with much smoother radiation patterns, and with less backward radiation. In short, the antennas are both more compact and more efficient when made with a Hi-Z surface.
The Hi-Z structure can be most easily understood by considering the effective circuit that describes the resonant cavities. In the structure shown in FIG. 1, the Hi-Z surface is constructed as a lattice of overlapping xe2x80x9cthumbtackxe2x80x9d-like protrusions on a flat metal ground plane 22. The protrusion consist of flat metal plates 10 connected to the ground plane by metal plated vias 13. This prior art structure shown here is built using printed circuit board techniques. The printed circuit board is not shown for ease of illustration, but the flat metal plates 10 would appear on the printed circuit board""s top surface while the ground plan 22 is disposed on its bottom surface. The capacitance of the structure is determined by the proximity and overlap area of the metal plates 10. The inductance is controlled by the area of the current loop that connects adjacent plates, which is primarily determined by the thickness of the structure. The resonance frequency of the surface is then given by   ω  =            1              LC              .  
Near the resonance frequency, the surface has high impedance, and can suppress the propagation of surface currents. The bandwidth of the surface, or the frequency band where the impedance is greater than 377 ohms, is given by   BW  =                              L          /          C                                                  μ            o                    /                      ϵ            o                                .  
This roughly determines the bandwidth of antennas that can be built on these surfaces.
Typically, in the prior art, Hi-Z surfaces are produced by printed circuit board techniques. In order to achieve a low resonant frequency ( less than 10 GHz or so) in a thin structure (a few mm thick), a large amount of built-in capacitance is required. This is accomplished using a multi-layer structure, in which the capacitors are of a parallel-plate geometry. The vias 12 are made by drilling through both boards, and then plating the inside of the holes with metal 13. The steps taken in fabrication are shown in side elevation in FIGS. 2(a)-2(f). First, two printed circuit boards, one relatively thick and one relatively thin form the starting materials (see FIG. 2(a)). The inner layers are patterned (see FIG. 2(b)), and the boards are bonded together (see FIG. 2(c)). Then holes 12 are drilled through the structure to define the positions of the vias (see FIG. 2(d) and the plan view of FIG. 2(g)). These are then plated with metal 13 (see FIG. 2(e)). Finally, the outer layers are patterned (see FIG. 2(f) and the plan view of FIG. 2(h)). The most time-consuming and expensive task is drilling the vias 12. A fast computer-controlled drill can drill on the order of one hole per second. Typical lattice periods for these structures are on the order of xc2xc inch, which means that the total drilling time can approach one hour per square foot.
What is needed is a method of producing a similar structure by faster and more economic techniques, in which the holes do not need to be drilled individually, but instead can be produced en masse by some other technique. This invention provides techniques for producing such a structure by molding, as well as new geometries that are amenable to such manufacturing techniques. The resulting structure is less expensive and less time-consuming to fabricate. Furthermore, it has the additional benefit that certain embodiments thereof can be tuned after fabrication to adjust for variations in the manufacturing process. This feature also allows a single mold to be used to build structures with slightly different resonant frequencies.
The present invention provides a Hi-Z surface that can be produced by injection molding, which permits large areas to be produced rapidly and at a low cost. Additionally, certain embodiments of the structure are also technically superior in that they can be tuned after manufacturing, to adjust for variations in the manufacturing process, thus allowing a single mold to be used for structures with slightly different resonance frequencies, and/or allowing different areas of a single Hi-Z surface to be tuned to different resonance frequencies.
In one aspect the present invention provides a method of making a high impedance surface comprising the steps of: molding a structure from a dielectric material to form the structure, the structure having a plurality of holes therein and a plurality of ridges on at least one major surface of the structure, the ridges having sidewalls; plating the structure, including the interiors of the holes therein and the sidewalls, with a layer of metal; removing at least a portion of the layer of metal which bridges across the ridges to thereby define capacitor plates on the sidewalls.
In another aspect the present invention provides a method of making a high impedance surface comprising the steps of: molding a structure from a dielectric material to form the structure, the structure having a plurality of holes therein and a plurality of trenches on at least one major surface of the structure, the trenches having sidewalls and bottom walls; and plating the structure, including the interiors of the holes therein and the sidewalls, but not the bottom walls of the trenches, with a layer of metal.
In still yet another aspect the present invention provides a method of making a high impedance surface comprising the steps of molding a structure from a dielectric material, the structure having a first major surface, a second major surface, a plurality of holes which penetrate both major surfaces, and a plurality of sidewall features on the first major surface; and applying at least one metal layer to the structure in the interiors of the holes therein, on the sidewall features, and on the second major surface, the at least one metal layers on the sidewall features defining plates of capacitors which are connected to neighboring plates of capacitors via the at least one metal plate in the holes and on the second major surface.
In still yet another aspect the present invention provides a method of making a high impedance surface comprising the steps of molding a structure from sheet metal, the structure having a plurality of openings therein with confronting sidewalls on the sides of the openings, the structure also having a plurality of protrusions projecting from a major surface thereof; and joining the structure to additional sheet metal such that ends of the protrusions remote from the major surface are coupled to the additional sheet metal.
In yet another aspect the present invention provides a high impedance surface comprising a molded structure having a repeating pattern of holes therein and a repeating pattern of sidewall surfaces, the holes penetrating the structure between first and second major surfaces thereof and the sidewall surfaces joining the first major surface; and a metal layer on the molded structure, the metal layer being disposed in or filling the holes, covering at least a portion of the second major surface, covering the sidewalls and portions of the first major surface to interconnect the sidewalls with other sidewalls via the metal layer on the second major surface and in the holes.